The removal of nitrogen and other contaminants from water is an important environmental task. Wastewater treatment plants in the developed world and in many developing countries oxidize NH4+ to NO3− before discharging the treated wastewater. This is done to decrease oxygen demand in the receiving waters.
Additionally, nitrogen excess in near shore environments has been identified as a major environmental problem leading to eutrophication and anoxia. Legislations are being implemented requiring the conversion of NO3− to N2 in conventional waste water treatment plants. The nitrogen compounds that are present in contaminated water, such as ammonium NH4+, nitrite (NO2−) and nitrate (NO3−) will have to be converted to elemental nitrogen N2, which can be released in the gaseous state into an open environment.
Nitrogen removal via nitrification and denitrification can be performed by microrganisms. Numerous studies have focused on the aerobic oxidation of ammonia or NH4+ to first nitrite (NO2−) and then nitrate (NO3−). Oxidation of NH4+ to NO2− is the first and rate limiting biological step in the nitrification, and also a required step prior to anaerobic NH4+ oxidation (anammox), which is the biological conversion of NH4+ and NO2− to nitrogen gas in the absence of oxygen. Anammox is the most common anaerobic NH4+ oxidation pathway in soils environments known so far, using NO2− as the electron acceptor. Anaerobic NH4+ oxidation is performed by anammox bacteria. Aerobic NH4+ oxidation is performed by two groups of organisms, ammonia-oxidizing bacteria (AOB), and ammonia-oxidizing archaea (AOA).
In the wastewater treatment plants, biological ammonium oxidation is conducted by aerobic nitrifying bacteria and requires aeration in the step with the highest energy input. An alternative is the partial nitrification (nitritation) Anammox system, which has been implemented in some treatment plants, with the goal of saving energy costs, since only half of the NH4+ is converted to NO2− aerobically. However, these Anammox based wastewater treatment systems need to operate between 28° C. and 35° C.
An NH4+ oxidation process coupled to iron (Fe) reduction called Feammox was identified (Clement J C et al. 2005 Soil Biol Biochem 37:2323-2328; Sawayama S. 2006; J Biosci Bioeng 101:70-72; Shrestha J et al. 2009 Soil Sci 174:156-164;Yang W H et al. 2012 Nat Geosc 5: 538-541, all of which are incorporated herein by reference as if fully set forth). Feammox is a process that can be described as the oxidation of NH4+ in the absence of molecular oxygen, with iron oxides [Ferric iron, Fe(III)] as the electron acceptor. In this reaction Fe(III) is reduced to Ferrous iron Fe(II), while NH4+ is transformed to NO2−, nitrogen gas (N2), or other nitrogen forms. Feammox may provide benefits for development of an improved system that does not require aeration or heating of the wastewater in temperate climates.